While you may be “welcome to use another mode of transportation,” bag searches aren’t really optional if Metro Transit Police follow you and deliberately make it known that they’re following you.

Quite so. The fact that Metro Police would follow you if you refused a bag search was first published on this blog a couple of years ago — “Taborn: bag search refusers will “be observed”” — with video footage of then-Metro Police Chief Michael Taborn’s comments to WMATA’s Riders Advisory Council (RAC):

In response to RAC member Diana Zinkl’s question about what would happen to people who refused a bag search and walked away, Chief Taborn responded:

CHIEF TABORN: Well I can tell you without any uncertainty that that person would be observed. And what that means to you is different than what it means to me, but that person would be observed.
DIANA ZINKL: Well could you clarify what ‘be observed’ means?
CHIEF TABORN: Be observed. Be, be observed. Be watched. [transcript]

That doesn’t make it any more acceptable now than it was then, of course. There’s also reason to believe the case law WMATA relies on —MacWade v. Kelly — doesn’t support following persons availing themselves of their 4th Amendment rights, since that ruling simply states that New York subway riders “may decline to be searched.” Period.

Exercising your Fourth Amendment rights at the Metro turnstile simply can not be — and must not be allowed to become — ‘probable cause’ for abridging your Fourth Amendment rights yet again as you turn away.